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      <a href="/2018/12/26/wxapp-watch/" title="小程序的奇技淫巧之 watch 观察属性" itemprop="url">小程序的奇技淫巧之 watch 观察属性</a>
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      <a href="https://godbasin.github.io" title="被删">被删</a>
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    <time datetime="2018-12-26T15:45:10.000Z" itemprop="datePublished">2018-12-26</time>
    更新日期:<time datetime="2018-12-26T15:46:14.638Z" itemprop="dateModified">2018-12-26</time>
    
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			<strong class="toc-title">文章目录</strong>
		<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#watch-观察属性使用"><span class="toc-number">1.</span> <span class="toc-text">watch 观察属性使用</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#安装依赖"><span class="toc-number">1.1.</span> <span class="toc-text">安装依赖</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#在-Component-中使用"><span class="toc-number">1.2.</span> <span class="toc-text">在 Component 中使用</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#在-Page-中使用"><span class="toc-number">1.3.</span> <span class="toc-text">在 Page 中使用</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#watch-观察属性实现"><span class="toc-number">2.</span> <span class="toc-text">watch 观察属性实现</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#watch-触发机制"><span class="toc-number">2.1.</span> <span class="toc-text">watch 触发机制</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#watch-监听更新机制"><span class="toc-number">2.2.</span> <span class="toc-text">watch 监听更新机制</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#参考"><span class="toc-number">2.3.</span> <span class="toc-text">参考</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#结束语"><span class="toc-number">3.</span> <span class="toc-text">结束语</span></a></li></ol>
		</div>
		
		<p>上一节我们介绍了小程序的 computed 计算属性，这次我们来讲讲 watch 观察属性叭~<br><a id="more"></a></p>
<h2 id="watch-观察属性使用"><a href="#watch-观察属性使用" class="headerlink" title="watch 观察属性使用"></a>watch 观察属性使用</h2><hr>
<p>这里我们直接先讲解下怎么用哇。</p>
<h3 id="安装依赖"><a href="#安装依赖" class="headerlink" title="安装依赖"></a>安装依赖</h3><p>整个实现的依赖包，放在 Github 上，大家可以去翻看和点星星：<a href="https://github.com/godbasin/watch-behavior" target="_blank" rel="external">watch-behavior</a>。</p>
<ul>
<li>安装<code>watch</code>：</li>
</ul>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">npm <span class="keyword">install</span> <span class="comment">--save miniprogram-watch</span></div></pre></td></tr></table></figure>
<blockquote>
<p>使用 behavior 需要依赖小程序基础库 2.2.3 以上版本，同时依赖开发者工具的 npm 构建。具体详情可查阅<a href="https://developers.weixin.qq.com/miniprogram/dev/devtools/npm.html" target="_blank" rel="external">官方 npm 文档</a>。</p>
</blockquote>
<h3 id="在-Component-中使用"><a href="#在-Component-中使用" class="headerlink" title="在 Component 中使用"></a>在 Component 中使用</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// 需要开发者工具 npm 依赖</span></div><div class="line"><span class="keyword">const</span> watchBehavior = <span class="built_in">require</span>(<span class="string">"miniprogram-watch"</span>);</div><div class="line"></div><div class="line">Component(&#123;</div><div class="line">  <span class="attr">behaviors</span>: [watchBehavior],</div><div class="line">  <span class="attr">properties</span>: &#123;</div><div class="line">    <span class="attr">propA</span>: &#123;</div><div class="line">      <span class="attr">type</span>: <span class="built_in">Number</span>,</div><div class="line">      <span class="attr">value</span>: <span class="number">0</span></div><div class="line">    &#125;</div><div class="line">  &#125;,</div><div class="line">  <span class="attr">data</span>: &#123;</div><div class="line">    <span class="attr">a</span>: <span class="number">0</span>,</div><div class="line">    <span class="attr">b</span>: &#123;</div><div class="line">      <span class="attr">c</span>: &#123;</div><div class="line">        <span class="attr">d</span>: <span class="number">33</span></div><div class="line">      &#125;,</div><div class="line">      <span class="attr">e</span>: [<span class="number">1</span>, <span class="number">2</span>, [<span class="number">3</span>, <span class="number">4</span>]]</div><div class="line">    &#125;</div><div class="line">  &#125;,</div><div class="line">  <span class="comment">// 可以将需要监听的数据放入 watch 里面，当数据改变时推送相应的订阅事件</span></div><div class="line">  <span class="comment">// 支持 data 以及 properties 的监听</span></div><div class="line">  watch: &#123;</div><div class="line">    propA(val, oldVal) &#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"propA new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    a(val, oldVal) &#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"a new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    <span class="string">"b.c.d"</span>: <span class="function"><span class="keyword">function</span>(<span class="params">val, oldVal</span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"b.c.d new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    <span class="string">"b.e[2][0]"</span>: <span class="function"><span class="keyword">function</span>(<span class="params">val, oldVal</span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"b.e[2][0] new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    <span class="string">"b.e[3][4]"</span>: <span class="function"><span class="keyword">function</span>(<span class="params">val, oldVal</span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"b.e[3][4] new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;</div><div class="line">  &#125;,</div><div class="line">  <span class="attr">methods</span>: &#123;</div><div class="line">    onTap() &#123;</div><div class="line">      <span class="keyword">this</span>.setData(&#123;</div><div class="line">        <span class="attr">a</span>: <span class="number">2</span>,</div><div class="line">        <span class="string">"b.c.d"</span>: <span class="number">3</span>,</div><div class="line">        <span class="string">"b.e[2][0]"</span>: <span class="number">444</span>,</div><div class="line">        <span class="attr">c</span>: <span class="number">123</span></div><div class="line">      &#125;);</div><div class="line">      <span class="comment">// 不在 data 里面的数据项不会放入观察者列表，比如这里的'b.e[3][4]'</span></div><div class="line">    &#125;</div><div class="line">  &#125;</div><div class="line">&#125;);</div></pre></td></tr></table></figure>
<h3 id="在-Page-中使用"><a href="#在-Page-中使用" class="headerlink" title="在 Page 中使用"></a>在 Page 中使用</h3><p><strong><code>Component</code>是<code>Page</code>的超集，因此可以使用<code>Component</code>构造器构造页面。</strong></p>
<p><a href="https://developers.weixin.qq.com/miniprogram/dev/framework/custom-component/component.html" target="_blank" rel="external">官方文档</a>：</p>
<blockquote>
<p>事实上，小程序的页面也可以视为自定义组件。因而，页面也可以使用<code>Component</code>构造器构造，拥有与普通组件一样的定义段与实例方法。但此时要求对应<code>json</code>文件中包含<code>usingComponents</code>定义段。</p>
</blockquote>
<ul>
<li><code>page.json</code></li>
</ul>
<figure class="highlight json"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">&#123;</div><div class="line">  <span class="attr">"usingComponents"</span>: &#123;&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<ul>
<li><code>page.js</code></li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// 这里我们就可以使用 Component 代替 Page</span></div><div class="line">Component(&#123;</div><div class="line">  <span class="attr">data</span>: &#123;</div><div class="line">    <span class="attr">a</span>: <span class="number">0</span>,</div><div class="line">    <span class="attr">b</span>: &#123;</div><div class="line">      <span class="attr">c</span>: &#123;</div><div class="line">        <span class="attr">d</span>: <span class="number">33</span></div><div class="line">      &#125;,</div><div class="line">      <span class="attr">e</span>: [<span class="number">1</span>, <span class="number">2</span>, [<span class="number">3</span>, <span class="number">4</span>]]</div><div class="line">    &#125;</div><div class="line">  &#125;,</div><div class="line">  <span class="comment">// 可以将需要监听的数据放入 watch 里面，当数据改变时推送相应的订阅事件</span></div><div class="line">  watch: &#123;</div><div class="line">    a(val, oldVal) &#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"a new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    <span class="string">"b.c.d"</span>: <span class="function"><span class="keyword">function</span>(<span class="params">val, oldVal</span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"b.c.d new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    <span class="string">"b.e[2][0]"</span>: <span class="function"><span class="keyword">function</span>(<span class="params">val, oldVal</span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"b.e[2][0] new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;,</div><div class="line">    <span class="string">"b.e[3][4]"</span>: <span class="function"><span class="keyword">function</span>(<span class="params">val, oldVal</span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(<span class="string">"b.e[3][4] new: %s, old: %s"</span>, val, oldVal);</div><div class="line">    &#125;</div><div class="line">  &#125;,</div><div class="line">  <span class="attr">methods</span>: &#123;</div><div class="line">    <span class="comment">// 页面的生命周期方法（即`on`开头的方法，如上面的`onLoad`），应写在`methods`定义段中。</span></div><div class="line">    onLoad() &#123;</div><div class="line">      <span class="comment">// 如访问页面`/pages/index/index?paramA=123&amp;paramB=xyz`，如果声明有属性(`properties`)`paramA`或`paramB`，则它们会被赋值为`123`或`xyz`</span></div><div class="line">      <span class="keyword">this</span>.data.paramA <span class="comment">// 页面参数 paramA 的值</span></div><div class="line">      <span class="keyword">this</span>.data.paramB <span class="comment">// 页面参数 paramB 的值</span></div><div class="line">    &#125;</div><div class="line">    onTap() &#123;</div><div class="line">      <span class="keyword">this</span>.setData(&#123;</div><div class="line">        <span class="attr">a</span>: <span class="number">2</span>,</div><div class="line">        <span class="string">"b.c.d"</span>: <span class="number">3</span>,</div><div class="line">        <span class="string">"b.e[2][0]"</span>: <span class="number">444</span>,</div><div class="line">        <span class="attr">c</span>: <span class="number">123</span></div><div class="line">      &#125;);</div><div class="line">      <span class="comment">// 不在 data 里面的数据项不会放入观察者列表，比如这里的'b.e[3][4]'</span></div><div class="line">    &#125;</div><div class="line">  &#125;</div><div class="line">&#125;);</div></pre></td></tr></table></figure>
<p>更多的，也可以参考本人的<a href="https://github.com/godbasin/wxapp-typescript-demo" target="_blank" rel="external">wxapp-typescript-demo</a>中的<a href="https://github.com/godbasin/wxapp-typescript-demo/tree/master/src/pages/watch" target="_blank" rel="external">watch page</a>，后续也会持续更新方便好用的能力 demo。</p>
<h2 id="watch-观察属性实现"><a href="#watch-观察属性实现" class="headerlink" title="watch 观察属性实现"></a>watch 观察属性实现</h2><hr>
<p>自定义组件中<code>computed</code>计算属性的实现，由<a href="https://github.com/wechat-miniprogram/computed" target="_blank" rel="external">官方</a>提供的。上一篇<a href="https://godbasin.github.io/2018/12/23/wxapp-computed/">《小程序的奇技淫巧之 computed 计算属性》</a>中，也有讲解大致思路和使用方法。</p>
<p>现在，轮到我们自己来实现一个<code>watch</code>观察属性了。</p>
<p>这里的实现主要也是针对自定义组件中的<a href="https://developers.weixin.qq.com/miniprogram/dev/framework/custom-component/behaviors.html" target="_blank" rel="external"><code>behaviors</code></a>。上一篇已经讲过，这里就不再复述啦。</p>
<h3 id="watch-触发机制"><a href="#watch-触发机制" class="headerlink" title="watch 触发机制"></a>watch 触发机制</h3><p>其实<code>watch</code>的触发机制，基本都在<code>setData</code>的时候触发。而在自定义组件里，会有两种情况需要需要触发对应的<code>watch</code>监听：</p>
<ul>
<li><code>properties</code>属性变化时</li>
<li><code>data</code>属性变化时（调用<code>setData</code>）</li>
</ul>
<h3 id="watch-监听更新机制"><a href="#watch-监听更新机制" class="headerlink" title="watch 监听更新机制"></a>watch 监听更新机制</h3><p>既然<code>properties</code>和<code>data</code>都需要监听，我们来整理下逻辑。大致流程如下：</p>
<ol>
<li>在组件初始化的时候，将对应的<code>watch</code>路径加进观察队列<code>observers</code>。</li>
<li>在<code>properties</code>和<code>data</code>属性变更时，触发更新。<ul>
<li><code>properties</code>可根据<code>observer</code>触发更新</li>
<li><code>data</code>可根据<code>setData</code>触发更新</li>
</ul>
</li>
<li>更新时，先对比变更路径，然后根据路径是否匹配（即<code>observers</code>是否存在对应观察者），来确定是否需要通知相应的观察者。</li>
<li>确定存在变更路径，则对比新数据与旧数据是否一致，一致则拦截不做通知。</li>
<li>因为<code>watch</code>可能存在循环触发更新，对一次更新的最大通知次数做限制（这里限制5次）。</li>
</ol>
<p>具体的实现可以在<a href="https://github.com/godbasin/watch-behavior" target="_blank" rel="external">watch</a>中找到。</p>
<h3 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h3><ul>
<li><a href="https://developers.weixin.qq.com/miniprogram/dev/framework/custom-component/component.html" target="_blank" rel="external">Component构造器</a></li>
<li><a href="https://developers.weixin.qq.com/miniprogram/dev/framework/custom-component/behaviors.html" target="_blank" rel="external">behaviors</a></li>
</ul>
<h2 id="结束语"><a href="#结束语" class="headerlink" title="结束语"></a>结束语</h2><hr>
<p>Npm 包的发布，是一件简单也挺复杂的事情。简单的话，你可以发布一个单文件，不带任何构建等。复杂的时候，你需要写好demo、test、构建环境等等。<br>有些时候，自己亲自参与做一下，会让你更容易理解，这也是为什么我们偶尔需要造点轮子的原因。</p>
  
		
		<p style="margin-top:50px;">查看Github有更多内容噢：<a href="https://github.com/godbasin">https://github.com/godbasin</a></p>
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	<p style="text-align: center;margin-top: 10px;margin-bottom: 20px;">码生艰难，写文不易，给我家猪囤点猫粮了喵~</p>
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  		<p>作者：被删</p>
  		<p>出处：<a href="https://godbasin.github.io">https://godbasin.github.io</a></p>
  		<p>本文版权归作者所有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。</p>
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